Biaxially-oriented polyester films, e.g., polyethylene terephthalate, are well-known for high dimensional stability, moisture resistance, tear resistance, and low cost, and have achieved substantial commercial utility. Such films are commonly used in electrical insulation, as a backing for pressure-sensitive adhesive tapes, magnetic recording tapes, or abrasives, and in many other products.
For instance, U.S. Pat. No. 4,413,298 (Pecsok et al.) discloses a jacket for magnetic recording diskettes which is made from biaxially-oriented polyester film containing particles of a different polymer, providing a matte finish that aesthetically masks scratches and smudges. The reference discloses incorporation of a pigment, e.g., carbon black, in the polyester to impart desired opacity to the film.
U.S. Pat. No. 3,579,609 (Sevenich), discloses that blending minor amounts of fusible heat-stable polymers of a lower-mono-alpha olefin (e.g., polyethylene, polypropylene, polybutylene) or tetramethylene oxide into polyester resin which is extruded into a film and thereafter oriented, imparts improved flex resistance to the resultant film.
U.S. Pat. No. 3,640,944 (Seppala et al.) discloses tape comprising a blend of 1) a major portion of polyethylene terephthalate polymer, 2) from about 1 to 30 percent by weight of a thermoplastic, high temperature, opacifying additive polymer, e.g., polysulfone and poly(4-methyl, 1-pentene), dispersed throughout the tape in discrete microscopic particles, and 3) an effective amount of a light absorbing material.
None of these references teaches that the films taught therein exhibit substantial side-to-side variation in color appearance.